Refs: #3874

Kamaki.clients.astakos.AstakosClient is now a high-level client lib that
offers access to token-authenticated astakos-aquired cached information.
Users who need an astakos client should use the snf-astakosclient instead.

Usage:

 astakos_cache = AstakosClient(<astakos/base/url/with/identity/api>)
 astakos_cache.authenticate(<token>)
 user_info = astakos_cache.user_info()
 compute_endpoints = astakos_cache.get_service_endpoints('compute', 'v2')

Major changes:

Modify ReST call from /im/authenticate to /tokens

New methods as AstakosClient instance cache accessors:
 get_services
 get_service_details
 get_service_endpoints

Method renaiming for existing AstakosClient instance cache accessors:
 info --> user_info
 user --> list_users